Amount of work covered by colleague is a sensitive reaction (7)
I believe the answer is:
allergy
'a sensitive reaction' is the definition.
(allergic reaction)
'amount of work covered by colleague' is the wordplay.
'amount of work' becomes 'erg' (scientific unit for work or energy).
'covered by' is an insertion indicator.
'colleague' becomes 'ally' (ally can mean a colleague or associate).
'erg' inserted into 'ally' is 'ALLERGY'.
'is' is the link.
(Other definitions for allergy that I've seen before include "Hypersensitive reaction to certain substances" , "Abnormal body reaction" , "Pathological aversion" , "Strong aversion" , "Unfavourable reaction to, say, pollen" .)